1. Field of the Invention
This invention relates to the field of video test equipment, for example for use by video production facilities. The invention produces a simultaneous display of three of four possible characteristics defining color video signals. Preferably, the invention concerns an improved form of vectorscope display including representation of luminance, saturation and hue in a three dimensional presentation which can be rotated at the operator's option for examining the character of video signals, particularly test patterns. The invention is also applied to two dimensional projection of a three dimensional display including R-Y and B-Y color difference together with luminance (Y); or red, blue and green amplitude; or any three dimensions that in combination define a video signal at a point in time; or any two of three variables in these sets as a function of time.
2. Prior Art
Color video signals are encoded and broadcast in a manner requiring three variables to fully characterize the desired color display at a point in time, i.e., at a given point in the scanned field or raster. Alternatively or as required for various steps in the generation, transmission and/or display of the video signal, the three variables may represent, for example, luminance, saturation and hue; or red, green and blue amplitude; or luminance (Y) and color difference signals (R-Y and B-Y). In conventional test equipment for video signals, it is necessary to use at least two separate displays to represent all three of the variables at a point in time. Typically, a vectorscope or polar display indicates the saturation and hue, while luminance is displayed orthogonally as a function of time or against hue phase angle, using a separate two dimensional display. A dual two dimensional display configuration adequately displays up to four variables, but of course requires the operator to view the displays separately and involves the expense of two substantially complete sets of display apparatus.
One mapping of color space can be defined as the locations in three orthogonal dimensions of luminance, saturation and hue. Luminance, or brightness, is the level of light energy perceived by the viewer, varying from dim to bright. Saturation or vividness is a measure of how pale or intense the "color" portion of a signal is, independent of luminance. Hue is is the dominant wavelength or color of the light. Luminance is encoded in the video signal by the DC offset, saturation by the peak to peak amplitude of a subcarrier modulated onto the luminance, and hue by the phase angle of the subcarrier relative to a burst reference. Another color space can be defined as the locations in orthogonal dimensions of red, blue and green light, to represent the full spectrum of colors. The variables each vary in time.
According to the color difference method of encoding video signals, the primary color level signals, red, blue and green, are transformed into a format where the luminance (Y) information is transmitted separately and two color difference signals (R-Y and B-Y) encode the chrominance. This conversion can be made by a resistive matrix which algebraically combines proportions of each primary color signal to develop the chrominance and luminance. Inasmuch as the eye perceives different colors to inherently have different brightness, particular percentages are used in this transformation.
Luminance, for example, is 30% red, 59% green and 11% blue, reflecting the apparent brightness of the respective colors. In a color bar test pattern display for testing the accuracy of video equipment, the primary colors as well as colors resulting from combinations of the primary colors are shown as vertical bars positioned in luminance order across the display. The colors, in decreasing luminance order are yellow, cyan, green, magenta, red and blue (white and gray bars on the left, and black on the right, are added for reference). During every horizontal scan, the video signal thus progresses through the colors in decreasing luminance order. The hue phase angles of the colors do not correspond to their luminance order. In other words, the color bar display is not in order of wavelength, as in a rainbow. As a result a vectorscope display of a color bar test pattern (i.e., a polar display as a function of hue phase angle) does not form a regular polygonal shape, instead producing a characteristic, but irregular star-like shape. The display may be quite complicated when displaying a video signal that has features other than regular colored bars.
Standardized displays such as color bar test patterns are used to test the accuracy of video encoding, transmission and display. The color bar test pattern is quite useful as it includes information on luminance levels, chrominance levels, luminance to chrominance amplitude ratio, hue values, primary color values, and transition timing between luminance and chrominance and between the two color difference signals R-Y and B-Y. It is useful to analyze the color bar test pattern signal with respect to these parameters at various points to verify the accuracy of video encoding and also to ensure that subsequent processing of the encoded signal has not distorted it. It is also useful to view the variation in the respective signals over time.
U.S. Pat. Nos. 4,488,168--Mino and 3,614,304--Schonfelder disclose the application of a vectorscope to analysis of a color bar test pattern. A reference generator is locked to the burst. The video signal is separated into B-Y and R-Y signals by quadrature phase demodulation, and the demodulated B-Y and R-Y signals are applied as the X and Y inputs to a CRT. With reference to Mino, the resulting two dimensional display for a color bar test pattern has dots or patches for yellow and red in a first quadrant, magenta in the second, blue and cyan in the third and green in the fourth. As seen in Schonfelder the dots or patches are connected by lines due to the repetitive scanning of the vectorscope beam which occurs during each horizontal scan of the raster in the color test pattern signal as the displayed point moves with the progress of the scanning point of the raster through the respective color bars. The saturation of the signal is represented by the radius at each point on the display. (Saturation =SQRT[(R-Y)2+(B-Y)2].). It is also possible according to the present invention to provide a vectorscope display wherein another variable (e.g., luminance) is effectively displayed orthogonally to the R-Y and B-Y axes. However due to the two dimensional nature of known displays it is not possible simultaneously to display three variables of interest.
U.S. Pat. No. 4,707,727--Penney discloses a so-called lightning display of video information. In a display of this type, one of the color difference signals represents the X axis component of a two dimensional display and the luminance is represented by the Y axis component. When displaying a color bar test pattern on such a device, the display appears as a zigzag line. Such a display can be used in conjunction with a standard vectorscope display, in which case the combination of the two displays can be examined with respect to three video variables, such as luminance, saturation and hue (or R-Y, B-Y, Y). However, in these two displays at least one of the variables appearing in each display also appears redundantly in the other display. Moreover, two substantially complete display apparatus are needed.
A lightning display format of luminance vs. color difference is similar to a graph of luminance vs. time, as can be obtained using a two dimensional display. Again, in order to display a full set of variables, the prior art uses two displays.
It is possible to display a three dimensional shape on a two dimensional display such as a CRT screen, by providing a two dimensional projection of the three dimensional shape. Computer assisted design (CAD) apparatus and the like are known wherein a three dimensional shape can be projected in isometric or perspective form on a two dimensional screen, or viewed from different perspectives in order to more clearly display a three dimensional shape in a way that is meaningful to humans. Typically, the display of a shape requires the generation of lines which are perceived as edges or surfaces of the shape. U.S. Pat. No. 4,754,269--Kishi et al discloses an arithmetic technique for converting coordinates in a three dimensional (XYZ) coordinate system or space, into projected coordinates on a two dimensional (XY) coordinate system, namely the surface of a flat display. According to Kishi, the angles of the individual axes relative to the edges of the display can be varied to change the perspective orientation, by plugging the desired angles into a trigonometric matrix conversion process.
According to the present invention, a polar format similar to a vectorscope is presented in a three dimensional form. This can be arranged according to the invention to provide a three dimensional network of lines at which the junctions of the lines correspond to the color information at different areas in a video signal. The apices appear as brighter points connected by relatively dimmer lines. The saturation and hue are shown as in a vectorscope display by the radius and phase angle of the displayed points. In addition, the luminance level at each point in the color bar test pattern is represented by a z axis or height level above a plane. The plane can be indicated by graticles if desired. The display as so configured is rotatable on at least one axis in the display under operator control for analyzing the video signal as a shaped surface. In viewing the presentation for a color test bar pattern, the display produces a plurality of points standing above a circular reference field (appearing elliptical when the display is rotated). The distance from the plane to the point represents luminance, and inasmuch as the luminance levels for the different colors vary, the color bar test pattern produces a standardized three dimensional body of dots at different apparent heights, connected by lines, in which the technician can readily compare all three video attributes to standard, using a single display. Similarly, according to the invention a simultaneous display of other three variable sets of data characterizing the video signal can be presented in the format of a two dimensional projection of three dimensional data, or the display can show two members of a three variable set against time.